1. Field of the Invention
This invention relates to highly compressible iron powder that is suitable for manufacturing electric and mechanical parts that require high magnetism and/or high mechanical strength by powder metallurgy.
2. Description of the Related Art
Powder metallurgy allows production of metallic parts having complicated shapes by near net shape forming and is widely used in production of various parts. Near net shape forming can readily produce target shapes without additional machining.
In powder metallurgy, metal powder such as iron powder having a desired particle size distribution is prepared by controlling the atomizing conditions for molten metal or the reduction conditions for metal oxide as a low material or by classifying powder particles through sieves. The controlled powder is mixed with a lubricant and another metal powder (or other metal powders) for forming an alloy, if necessary. The metal powder or metal powder mixture is compacted in a die and the resulting green compact is sintered or treated with heat to form a part. Alternatively, the metal powder or metal powder mixture is mixed with a binder such as resin and the mixture is compacted in a die.
Such powder metallurgy is employed in production of mechanical parts for use in vehicles and soft magnetic parts such as transformer cores and noise filter cores for eliminating noise in electronic circuits. Higher density is required to maintain high mechanical strength for mechanical parts and high permeability for magnetic parts. High compressibility must be required of iron powder to increase the density of the parts.
For example, Japanese Examined Patent Application Publication No. 8-921 (hereinafter referred to as JP-B2-8-921) discloses an iron powder having the following particle size distribution: On the bases of mass percent of fractions after sieve classification using sieves defined in Japanese Industrial Standard (JIS) Z 8801 (Ed. 1984), the iron powder contains 5% or less of −60/+83-mesh particles that pass through a sieve having a nominal opening of 250 μm and do not pass through a sieve having a nominal opening of 165 μm, 4% and more to 10% or less of −83/+100-mesh particles that pass through a sieve having a nominal opening of 165 μm and do not pass through a sieve having a nominal opening of 150 μm, 10% and more to 25% or less of −100/+140-mesh particles that pass through a sieve having the nominal opening of 150 μm and do not pass through a sieve having a nominal opening of 106 μm and 10% and more to 30% or less particles that pass through a 330-mesh sieve having a nominal opening of 45 μm. Furthermore, the crystal grain size in iron particles of the particle size of −60/+200-mesh that pass through a sieve having the nominal opening of 250 μm and do not pass through a sieve having a nominal opening of 75 μm grows large by the grain size number of 6.0 or less according to a method for measuring a ferrite particle size defined in JIS G 0552 (Ed. 1977). According to JP-B2-8-921, high-density parts are obtained from such a pure iron powder.
The resulting iron powder is compounded with 0.75% zinc stearate as a powder metallurgy lubricant and the resulting compound is compacted under a compacting pressure of 490 MPa. However, the density of the green compact is 7.08 to 7.12 g/cm3 (7.08 to 7.12 Mg/m3). When this pure iron powder is used in magnetic parts such as magnetic cores, the parts do not have satisfactorily high flux density and permeability. Accordingly, the green density is still insufficient.
Nowadays, iron powder metallurgy parts must have higher strength to reduce the volume and weight of mechanical parts for vehicles. In general powder metallurgy, high-strength parts are produced by a double-press double-sintering method including a first compaction and sintering step and a second compaction and sintering step. Alternatively, the high-strength parts are produced by a sinter forging process including a compaction and sintering step and a hot forging step. Unfortunately, these processes increase production costs.
It would, therefore, be advantageous to provide a highly compressible iron powder suitable for production of magnetic parts having excellent magnetic characteristics and mechanical parts having high mechanical strength.